Epithelia are tissues of which the cells are joined to and interlinked with one another and lie on a basal membrane. They form either an external covering, for example at the surface of the skin, or the epidermis, or an internal covering, at the surface of a mucosa. They can also form glands.
More specifically, these epithelia are structures of which the homeostasis results from the implementation of a finely regulated set of intracellular and extracellular signals acting at all the stages of cell proliferation, migration and differentiation, and also of the synthesis of the various extracellular matrix components. These signals can in particular result from the action of factors produced by keratinocytes.
The maintaining of the correct physiological functions of an epithelium involves, in particular, epithelial terminal differentiation and/or proteoglycan synthesis.
As regards more particularly the epidermis, it is an epithelium, conventionally divided up into a basal layer of keratinocytes containing, in particular, skin stem cells and constituting the germinative layer of the epidermis, a “spiny” layer constituted of several layers of polyhedral cells placed on the basal layer, a “granular” layer comprising one to three layers said to be of flattened cells containing distinct cytoplasmic inclusions, keratohyalin granules, and finally, a set of upper cells, called horny layer (or stratum corneum) constituted of keratinocytes at the terminal stage of their differentiation, called corneocytes.
The stratum corneum, the outermost part of the skin which performs the function of a barrier between the organism and the environment, and the hair shaft, the emerging part of the hair follicle which constitutes the head of hair, both represent the result of the keratinocyte differentiation process. Epidermal differentiation follows a process of maturation in which keratinocytes from the basal layer differentiate and migrate so as to result in the formation of corneocytes, which are completely keratinised dead cells. This differentiation is the result of perfectly coordinated phenomena which will result in the thickness of the epidermis being kept constant and thus ensure the homeostasis of the epidermis.
Many skin disorders or pathological conditions can result from a dysfunction of epidermal homeostasis.
Thus, in the case of aged skin, this dysfunction is generally manifested through the appearance of wrinkles (microrelief and deep wrinkles), a loss of elasticity, a rough feel and dryness. From the histological point of view, a flattening of the dermo-epidermal junction and a decrease in thickness of the dermis and of the epidermis are observed. The collagen and glycosaminoglycan content decreases. The barrier function of the skin is impaired. All these phenomena are increased by chronic exposure to the sun.
Similarly, this dysfunction may be worsened in women during the menopause.